(1) Field of the Invention
The present invention relates to a relative car positioning system using car communication which exchanges information among nearby cars in order to support smooth and safe traffic and especially to a relative car positioning system using car communication which cancels an error of GPS wave propagation time and can accurately calculate a relative position of a nearby running car to the own car position.
(2) Description of the Prior Art
An apparatus built in a car communication application system used to support smooth and safe traffic is known as a usual relative car positioning system using car communication, as being disclosed, for example, in Japanese Patent Laid-Open 4-297000 (car group information collecting system by car communication), Japanese Patent Laid-Open 5-266399 (running control apparatus by car communication, etc.
An example of a relative car positioning system using car communication in accordance with the prior art is explained below, referring to the drawing.
FIG. 3 is a block diagram of a relative car positioning system using car communication in accordance with the prior art. A relative car positioning system is composed of a car communication transceiver 1, GPS receiving means 2, GPS information transmission/reception means 8 and relative positioning means by absolute position difference 9.
The function of a relative car positioning system using car communication in accordance with the prior art is explained below.
As a car communication transceiver 1, a transceiver module communicating data with a nearby running car is used, for example, using a millimeter band wave (30 to 300 GHz).
GPS receiving means 2 receives a wave transmitted from a plurality of GPS satellites in the sky and obtains wave propagation times from each GPS satellite to GPS receiving means 2, the position of each GPS satellite in the sky and the absolute position (for example, taking the center of the earth as an origin) of the GPS receiving means (in the own car) from the above information.
GPS information transmission/reception means 8 gives the own car position obtained at GPS receiving means 2 to car communication transceiver 1 as data for transmission, and car communication transceiver 1 transmits the data to every car running nearby. On the other hand, GPS information transmission/reception means 8 obtains an absolute position of each car calculated at the GPS receiving means of each car from car information obtained from every car running nearby through car communication transceiver 1 as GPS information.
Although the absolute position of each car running nearby is important, relations of relative positions between the own car and the cars running nearby are more important. Because, for example, whether the cars collide or not can be predicted from the relations of the relative positions.
Relative car positioning means by absolute position difference 9 calculates a relative position of the car running nearby to the own car position from the difference between the absolute position of each car calculated at GPS receiving means of each car and the absolute position of the own car obtained at GPS receiving means 2.
According to the above composition, the relative position of the car running nearby to the own car position can be known in real time and a relative car positioning system using car communication can be realized in which wide application such as safety and smoothness in traffic can be expected.
Although another method, such as using a car radar, can be used to obtain a relative distance, it is almost impossible to measure if an obstacle, such as the third car, comes to enter between the own car and the car running nearby. However, if a relative car positioning system using car communication is used, there is an advantage in which a position of the car running nearby, which is hard to measure even by a radar, can be obtained by using, for example a car running just ahead of the own car as a transponder.
In a relative car positioning system using car communication composed like the above, because the absolute position of each car is calculated independently at GPS receiving means of each car running nearby, an error occurs in the relative position of each car due to an error included in a GPS wave propagation time. As a result, the relative position of each car running nearby to the own car position is influenced by the error and a relative position with a good accuracy can not be obtained. This is a problem.
For example, in the case of not having compensation by a base station like DGPS, the position accuracy using a GPS cannot keep an accurate measurement of wave propagation time due to errors included in information transmitted from the GPS, and could have an absolute position error of up to 150 meters. Then, at worst, an error of 300 meters could occur as a relative position between two cars. Thus, it cannot be used at all for safety purposes, such as providing collision warnings.
Further, it sometimes could not be used for compensating a relative distance obtained by the other method, such as a car radar (which can usually detect with an accuracy of several meters if the target can be caught), because the accuracy is too bad.
The present invention is characterized by providing relative positioning means, utilizing GPS propagation time difference, which obtains a relative position of a car running nearby to the own car position by transmitting and receiving a wave propagation time from a GPS satellite to each car running nearby, and by transmitting and receiving information (ID or the like) of each of the GPS satellites. Not by transmitting and receiving absolute position data calculated independently among each of the cars. The differences between the GPS wave propagation time at each car running nearby and the GPS wave propagation time at the own car is calculated by solving simultaneous equations expressing relations between more than three pieces of GPS wave propagation time differences and a relative position as an unknown quantity.
Thus, the error of GPS wave propagation time is canceled and the relative position of the car running nearby to the own car position can be calculated with a good accuracy.